Insert and method of installation thereof

ABSTRACT

An insert for installation in a relatively thin or weak workpiece to provide an anchor for a self-tapping screw, which insert is in the form of an integral metal structure which comprises: a body portion; a head portion located at or near one end of the body portion and extending radially therefrom; a receptacle portion located at the other end of the body portion remote from the head portion; and a through bore extending throughout the head, body and receptacle portions; the interior bore of the receptacle portion being substantially unthreaded for the reception of a self-tapping screw; in which the exterior cross-sectional shape and dimensions of the receptacle portion, and the interior cross sectional shape and dimensions of the body portion are respectively such that the body portion is deformable, when the receptacle portion is drawn towards the head portion, to secure the insert in a suitable hole in a workpiece through which the receptacle and body portions have been inserted so that the head portion is adjacent the workpiece and a substantial length of the body portion protrudes beyond the workpiece; and in which any transverse dimension of the exterior of the receptacle portion is no greater than the corresponding transverse dimension of the interior of the body portion in the same transverse plane; the exterior cross-sectional shape and dimensions of the body portion being such that, when the receptacle portion is drawn towards the head portion, the part of the body portion nearer the receptacle portion enters inside the part of the body portion nearer the head thereby to enhance the engagement of the deformed body portion with the workpiece.

RELATED/PRIORITY APPLICATIONS

This application claims priority with respect to InternationalApplication No. PGT/GB00/04769, filed Dec. 13, 2000 and BritishApplication No. 9929560.2, filed Dec. 14, 1999.

The invention relates to an insert for installation in a relatively thinor weak workpiece to provide an anchor receptacle for a self-tappingscrew or the like.

Self-tapping screws are widely used in industry as providing a means ofsecuring together two components which is inexpensive in respect of bothmanufacturing cost and installation time. However, their use has anumber of disadvantages, stemming from the material or receptacle intowhich they are inserted.

When a self-tapping screw is driven directly into a workpiece of thin orweak material, the failure strength of the thread formed in theworkpiece is low. Further, there is little margin between this and thetorque that is required to start the thread engagement. Thus it can bedifficult to control the fastener installation torque so as to avoidthread stripping. Also, the tension strength of the resultant joint islimited by the tensile strip-out force of the thread in the thin or weakmaterial.

A grommet or insert of plastics material (e.g. nylon) is sometimes usedas a receptacle for a self-tapping screw. However, since plasticsmaterial is relatively weak, it requires a long length of screwengagement to provide enough strength to create sufficient marginbetween the screw-engagement torque and the thread strip-out torque soas to enable adequate control of the operation in a practical industrialenvironment, such as a car assembly line. The long length of the screwengagement produces a large protrusion on the rear or blind side of theworkpiece, which may be awkward or dangerous in the use of the assembledproduct. The longer screw also takes more time to instal.

Furthermore since the insert or anchor is of totally different materialto the workpiece (e.g. plastics as against steel), ultimate scrapping ofthe assembled product (e.g. a car body) makes it difficult to recoverboth materials. Plastics components are usually burnt off and thematerial not recovered.

One possible alternative would be to use a blind threaded insert, suchas is widely known and available in many countries under the name AVDELTHIN SHEET NUTSERT (Registered Trade Marks) in conjunction with amachine screw or bolt. This can have drawbacks under production-lineconditions where speed of operation is under pressure, such asdifficulty of engaging threads, leading to cross-threading. Further, ifthe component with the installed threaded insert is painted before thescrew is applied, the paint can enter the insert and jam the threadsthus preventing proper engagement of the screw.

One of the aims of the present invention is to provides an insert whichovercomes these practical disadvantages of the prior art. The inventionis defined in the accompanying claims.

Some specific embodiments of the present invention will now be describedby way of example and with reference to the accompanying drawings, inwhich:

FIGS. 1A and 1B, 2A and 2B, and 3, 4 and 5, illustrate two alternativeforms of insert;

FIGS. 6 to 10 illustrate the progressive stages in the deformation ofone form of insert as it is installed;

FIGS. 11 & 12 illustrate an installation tool for the repetitioninstallation of inserts of a second form;

FIGS. 13 to 18 illustrate the progressive stages in the deformation ofthe second form of insert as it is installed by the tool of FIGS. 11 &12; and

FIGS. 19 & 20 illustrate a self-tap screw being screwed into aninstalled insert.

Referring first to FIGS. 1 to 5, FIGS. 1A & 2A are elevations of twoslightly different forms of insert, FIGS. 1B & 2B are underplan views inthe direction of the arrows IB & IIB of FIGS. 1A & 2A, and FIGS. 3, 4 &5 are sections on the lines III—III, IV—IV and V—V of FIGS. 1A & 2A. InFIGS. 1A & 2A the walls of the bore are indicated in phantom.

The first example insert 11 shown in FIGS. 1, 3 & 4 is a unitarystructure of steel of generally tubular form with a through bore 13. Itcomprises a body portion 14 formed integrally with a radially enlargedflange-like head 15 at one end and a receptacle portion 16 at the other.The exterior 17 and interior 18 are both substantially square in shape,having chamfered or rounded corners 19. The receptacle 16 has itsexterior 21 and interior 22 both cylindrical, i.e. circular incross-section, the interior face 22 being unthreaded. The axial lengthsof the body 14 and receptacle 16 are substantially the same. The head 15is circular in exterior shape and relatively thin in the axialdirection. The wall thickness of the body 14 is similar to the thicknessof the head flange 15, whereas the wall thickness of the receptacle 16is substantially greater than these. The diameter of the exterior 21 ofthe receptacle 16 is slightly smaller than the minimum diameter (i.e.the distance between opposed faces) of the interior 18 of the body, sothat the exterior 21 of the receptacle 16 is a sliding fit in theinterior 18 of the body 14 of an identical insert 11.

The second form of insert, illustrated in FIGS. 2A, 2B, 3 and 5 is amodification of that first described with reference to FIGS. 1A, 1B, 3and 4, like parts being indicated by like numerals. The second form ofinsert 23 is identical, except that the exterior 24 of its receptacle 25is of substantially square cross-section, with rounded corners 26.Again, the exterior 24 of the receptacle 25 of the insert is a slidingfit within the interior of the body 14 of an identical insert.

FIGS. 6 to 10 illustrate the installation sequence of the first form ofinsert 11 shown in FIGS. 1A and 1B. The installation tool 26 isgenerally similar to those well-known for installing blind breakstemrivets, except that it has a re-usable mandrel 27 comprising anelongated stem 28 with gripping grooves 29 adjacent one end and anenlarged head 31 at the other end, including a taper 32 by which itmerges into the stem 28. The diameter of the mandrel stem 28 is asliding fit within the interior 22 of the receptacle 16 of the insert.As is usual the tool has a tubular nosepiece 33 ending in an annularanvil face 34. Inside the nose piece 33 is a jaw-carrier 35 holding twojaws 36 which engage the gripping grooves 29 of the mandrel. The jawcarrier provides a tapered collet which urges the jaws together. Inorder to facilitate the installation of the insert, the tool alsoincludes a reciprocable support tube 37, the exterior of which is of thesame diameter as the exterior of the insert receptacle 16, so that it isa sliding fit within the body 14 of the insert 11, and its interior is asliding fit on the mandrel stem 28. The tool includes means (such aspneumatic, hydro-pneumatic or electric drive), not shown, for retractingthe jaw carrier 35 with respect to the nosepiece 33.

In order to use the tool, the mandrel is removed from the tool bypushing it inwards to open the jaws, and then withdrawing it. An insertis assembled over the tail end of the mandrel, receptacle end first,until the end of the receptacle contacts the bottom of the taper 32 ofthe mandrel head, (see FIG. 6). The mandrel tail is then insertedthrough the support tube 37, to engage within the jaws 36. The relativedimensions of the insert and tool are such that the insert head 15 isthen in contact with the tool anvil 34 (FIG. 6). In order to instal theinsert 11 in an appropriately dimensioned pre-formed square hole 38 in asheet-like workpiece 39, the tool 26 is manipulated by its operator (ifthe tool is hand-held, or it could be guided and advanced mechanically)so that the insert 11 is fed into the hole 38 until the insert head 15is in contact with the workpiece 39 against which it is supported by theanvil 34. As illustrated in FIG. 6, the majority of the length of thebody 14 of the insert protrudes beyond the workpiece.

The tool is then activated so that its drive retracts the jaw carrier 35within the nosepiece 33, pulling the mandrel head 31 towards the anvil34. The axial compression thus exerted on the insert causes it to deformprogressively in two ways.: The mandrel head taper 32 and full diameterportion 31 progressively radially expand the receptacle 16 and enter itaxially, and the body 14 progressively collapses axially by outwardradial expansion. The progressive stages are illustrated in FIGS. 7 to10. The body 14 of the insert buckles outwardly (FIG. 7), which bucklingis promoted by the shape of the junction between the body 14 and thereceptacle 16. Eventually the buckling is sufficient to form a foldedflange 41 (FIG. 8), the workpiece 39 being gripped between the inserthead 15 and the flange 41.

During the initial stage of its collapse, the interior of the body 14 ofthe insert is supported by the support tube 37, so that the collapseoccurs symetrically, thus providing accurate positioning of thereceptacle with respect to the hole and perpendicular to the plane ofthe workpiece. The support tube is withdrawn from the axially collapsingbody, first by urging from the body 14 and later by the mandrel headtaper 32.

As the mandrel head advances through the receptacle 16 of the insert, itdoes two things. Firstly, it radially expands the part of the body 14which is adjacent the head 15 and inside the hole 38 in the workpiece 39into tight contact with the wall of the hole, thus increasing thesecurity of installation of the insert in the workpiece. This alsoallows for a substantial tolerance in the size of the hole 38 preformedin the workpiece.

Secondly, it forms the enlarged inside wall of the bore of thereceptacle to an accurately known diameter, thus dimensioning the boreof the installed receptacle with a close tolerance to the optimumdiameter to receive the intended self-tap screw, and therefore goodcontrol of the tolerance needed to drive the screw. FIG. 10 shows theinsert installed in the workpiece after the tool has been removed.

The mandrel 28 can be re-used for installing a large number of inserts,one at a time. However, a much more efficient method is to use arepetition system, in which a number of inserts can be loaded on thestem of a long mandrel to form a column of inserts which are advanced,as each insert is placed in turn. A suitable tool for doing this isillustrated in FIGS. 11 and 12, in which FIG. 11 is an axiallongitudinal section through the nosepiece end of the tool and FIG. 12is a cross-section on the line XII—XII of FIG. 11. Like parts to thosein the tool of FIG. 6 are shown by like numbers.

The tool of this example is substantially identical with the well-knowntools used for repetition installation of the tubular blind rivets whichare well-known and widely available in many countries under theRegistered Trade Marks CHOBERT and BRIV. The construction and operationof the tool need not be here further described.

As illustrated in FIG. 11, the fact that the exterior of the receptacleof each insert is a sliding fit within the body part of an identicalinsert provides two advantages. Firstly, this allows telescoping of theinserts on the mandrel, with the exterior of the receptacle 25 of eachinsert fitting within the body 14 of the insert next in front. Thisincreases the number of inserts which can be pre-loaded on the long stem42 of the repetition mandrel 43, the head 44 of which is drawn throughthe leading insert 45. The column of inserts is resiliently urgedforwards along the reciprocable mandrel. When the mandrel is retracted,the head of the leading insert abuts against the nosepiece anvil 34, andthe insert is deformed and installed in the same way as previouslydescribed. The second advantage is that during deformation and collapse,each insert body is supported and positioned by the receptacle of thenext insert, instead of by the supporting tube 37 of the single-use tooldescribed above. Support for the body portion of the last insert of thecolumn is provided by a suitable tubular projection on the pusher orspring-follower (not shown) which urges the inserts forward.

As in the CHOBERT and BRIV systems, successive inserts are fed forwardsone at a time through nosepiece jaws 46, which close behind the leadinginsert and grip the next following insert. The construction of the toolallows the use of a further feature of the invention. It will berecalled that the second form of the insert, shown in FIGS. 2A and 2B,has the exterior of its receptacle portion 25 of square shape. Thus whenthe square receptacle of one insert is received within the sliding-fitbody of the next, rotational alignment of that next insert about themandrel axis is defined and maintained. To this end, as illustrated inFIG. 12 the nosejaws 46 of the tool have interior faces which togetherform a square cross-section 47, corresponding to the squarecross-section of the exterior of the insert body 14. Thus the insertheld by the nosepiece jaws 46 is rotationally orientated by the jaws,and the leading insert which is about to be installed is rotationallyorientated by the receptacle of the aforementioned insert behind itwithin its body. Thus, whether the installation tool is hand-held ormachine supported and guided, accurate orientation of the insert to beinstalled, with the preformed square hole in the workpiece can beachieved.

FIGS. 13 to 18 are generally similar to FIGS. 6 to 10, but show insmaller increments the progress of deformation of the insert, thesupporting receptacle of the next insert being omitted for clarity ofillustration, and the nosejaws being shown in simplified form.

FIG. 13 shows the relationship between the insert 45, the mandrel 43 andthe nosetip anvil 34 of the placing tool at the commencement of theinstallation operation when the pulling force applied to the mandrel islow. As the pulling force on the mandrel increases in magnitude, themandrel head 44 begins to deform and radially expand the receptacle 25of the insert. At the same time the body 14 “bulbs” whilst axiallycontracting. Because the exterior diameter of the receptacle 25 at itsjunction with the body 14 is of a smaller diameter than the exteriordiameter of the body, the line of force as it reacts through the bodywall is offset. This provides a moment of force which acts to urge thebody wall to expand radially outwards as shown in FIG. 14.

As more of the mandrel head progressively enters the receptacle 25,there comes a point there the force required to cause further expansionof the receptacle by the mandrel head 44 is sufficiently great to causethe body portion to collapse axially and expand radially, as can be seenin FIG. 15.

This continues until the deformed base of the body contacts the deformedbase of the inner wall of the body as shown in FIG. 16.

As the pulling force of the mandrel increases, the mandrel head 44 isdrawn through the bore of the receptacle 25, expanding it and furtherdeforming the base of the body 14 into a conical shape. The outersurface of the cone is pressed hard against the deformed inner wall ofthe body, as shown in FIG. 17, which in turn forces the outer deformedsurface of the body hard against the workpiece. This has two purposes;it provides an installed insert which is very securely fixed to theworkpiece with no risk of looseness or rattling, and secondly, wherethere is a large variation in the hole size, it always ensures that theexpansion of the body of the insert is sufficient to fill the hole inthe workpiece.

The final installed configuration of the insert is shown in FIG. 18.This shows that the conical deformed shape of the base of the body ispositioned relative to the body wall in such a way that it supports thebody wall and therefore adds strength to the installed insert. It alsomeans that the receptacle portion is held rigid in an axial and radialdirection relative to the head of the insert.

FIGS. 19 & 20 illustrate the insertion of a self-tap screw 48 into theinstalled receptacle 25 to attach a component 49 to the workpiece. Theaction of tightening the self-tapping screw 48 will further force theconical face against the interior face of the deformed body to maintainthe rigidity of the insert.

In the foregoing example, the insert is made of low carbon steel.Typically the thickness of the wall of the body portion 14 would be 0.4mm.

The invention is not restricted to the details in the foregoingexamples. For instance, the non-circular shapes need not be square butcould be, for example, hexagonal.

What is claimed is:
 1. An insert for installation in a relatively thinor weak workpiece to provide an anchor for a self-tapping screw, whichinsert is in the form of an integral metal structure which comprises: abody portion; a head portion located at or near one end of the bodyportion and extending radially therefrom; a receptacle portion locatedat the other end of the body portion remote from the head portion; and athrough bore extending throughout the head, body and receptacleportions; the interior bore of the receptacle portion beingsubstantially unthreaded for the reception of a self-tapping screw; inwhich the exterior cross-sectional shape and dimensions of thereceptacle portion, and the interior cross-sectional shape anddimensions of the body portion are respectively such that the bodyportion is deformable, when the receptacle portion is drawn towards thehead portion, to secure the insert in a suitable hole in a workpiecethrough which the receptacle and body portions have been inserted sothat the head portion is adjacent the workpiece and a substantial lengthof the body portion protrudes beyond the workpiece; and in which anytransverse dimension of the exterior of the receptacle portion is nogreater than the corresponding transverse dimension of the interior ofthe body portion in the same transverse plane; the exteriorcross-sectional shape and dimensions of the receptacle portion, and theinterior cross-sectional shape and dimensions of the body portion beingsuch that, when the receptacle portion is drawn towards the headportion, the part of the body portion nearer the receptacle portionenters inside the part of the body portion nearer the head thereby toenhance the engagement of the deformed body portion with the workpiece.2. An insert for installation in a relatively thin or weak workpiece toprovide an anchor for a self-tapping screw, which insert is in the formof an integral metal structure which comprises: a body portion; a headportion located at or near one end of the body portion and extendingradially therefrom; a receptacle portion located at the other end of thebody portion remote from the head portion; and a through bore extendingthroughout the head, body and receptacle portions; the interior bore ofthe receptacle portion being substantially unthreaded for the receptionof a self-tapping screw; the body portion being deformable, when thereceptacle portion is drawn towards the head portion, to secure theinsert in a suitable hole in a workpiece through which the receptacleand body portions have been inserted so that the head portion isadjacent the workpiece and a substantial length of the body portionprotrudes beyond the workpiece; the exterior cross-sectional shape anddimensions of the receptacle portion being smaller than the interiorcross-sectional shape and dimensions of the body portion, such that thereceptacle portion of the insert is a sliding fit within the bodyportion of an identical insert.
 3. An insert as claimed in claim 2wherein the exterior cross-sectional shape and dimensions of thereceptacle portion, and the interior cross-sectional shape anddimensions of the body portion are respectively such that the bodyportion is deformable, when the receptacle portion is drawn towards thehead portion, to secure the insert in a suitable hole in a workpiecethrough which the receptacle and body portions have been inserted sothat the head portion is adjacent the workpiece and a substantial lengthof the body portion protrudes beyond the workpiece; and in which anytransverse dimension of the exterior of the receptacle portion is nogreater than the corresponding transverse dimension of the interior ofthe body portion in the same transverse plane; the exteriorcross-sectional shape and dimensions of the receptacle portion, and theinterior cross-sectional shape and dimensions of the body portion beingsuch that, when the receptacle portion is drawn towards the headportion, the part of the body portion nearer the receptacle portionenters inside the part of the body portion nearer the head thereby toenhance the engagement of the deformed body portion with the workpiece.4. An insert as claimed in claim 2, in which both the exteriorcross-sectional shape of the receptacle portion and the interiorcross-sectional shape of the body portion are substantiallynon-circular.
 5. An insert as claimed in claim 2, in which the interiorcross sectional shape of the body portion is substantially non-circularand the exterior cross-sectional shape of the receptacle portion issubstantially circular.
 6. An insert as claimed in claim 4, in which theexterior cross sectional shape of the body portion of the insert is alsosubstantially non-circular.
 7. A plurality of inserts in combinationwith a pull-through mandrel for installing the inserts in succession,each insert being for installation in a relatively thin or weakworkpiece to provide an anchor for a self-tapping screw, which insert isin the form of an integral metal structure which comprises; a bodyportion; a head portion located at or near one end of the body portionand extending radially therefrom; a receptacle portion located at theother end of the body portion remote from the head portion; and athrough bore extending throughout the head, body and receptacleportions; the interior bore of the receptacle portion beingsubstantially unthreaded for the reception of a self-tapping screw; thebody portion being deformable, when the receptacle portion is drawntowards the head portion, to secure the insert in a suitable hole in aworkpiece through which the receptacle and body portions have beeninserted so that the head portion is adjacent the workpiece and asubstantial length of the body portion protrudes beyond the workpiece;the exterior cross-sectional shape and dimensions of the receptacleportion being smaller than the interior cross-sectional shape anddimensions of the body portion, such that the receptacle portion of theinsert is a sliding fit within the body portion of an identical insert;the mandrel comprising an elongated stem and an enlarged head, theplurality of inserts being loaded on the stem with the receptacleportion of each insert facing towards the mandrel head and receivedwithin the body portion of the adjacent insert.
 8. The combination asclaimed in claim 7, in which both the exterior cross-sectional shape ofthe receptacle portion, and the interior cross-sectional shape of thebody portion, of each insert are substantially non-circular.
 9. Thecombination of a plurality of inserts with a mandrel as claimed in claim8 in which each substantially non-circular shape is substantiallysquare.
 10. An insert installation tool for installing a plurality ofinserts, each insert being for installation in a relatively thin or weakworkpiece to provide an anchor for a self-tapping screw, which insert isin the form of an integral metal structure which comprises: a bodyportion; a head portion located at or near one end of the body portionand extending radially therefrom; a receptacle portion located at theother end of the body portion remote from the head portion; and athrough bore extending throughout the head, body and receptacleportions; the interior bore of the receptacle portion beingsubstantially unthreaded for the reception of a self-tapping screw; thebody portion being deformable, when the receptacle portion is drawntowards the head portion, to secure the insert in a suitable hole in aworkpiece through which the receptacle and body portions have beeninserted so that the head portion is adjacent the workpiece and asubstantial length of the body portion protrudes beyond the workpiece;the exterior cross-sectional shape and dimensions of the receptacleportion being smaller than the interior cross-sectional shape anddimensions of the body portion, such that the receptacle portion of theinsert is a sliding fit within the body portion of an identical insert,and wherein both the exterior cross-sectional shape of the receptacleportion and the interior cross-sectional shape of the body portion aresubstantially non-circular; which installation tool comprises thecombination of a plurality of such inserts together with a pull-throughmandrel, for installing the inserts in succession, the mandrelcomprising an elongated stem and an enlarged head, the plurality ofinserts being loaded on the stem with the receptacle portion of eachinsert facing towards the mandrel head and received within the bodyportion of the adjacent insert, and in which the exteriorcross-sectional shape of the body portion of each insert is alsosubstantially non-circular, the installation tool further comprisingmeans for engaging with the non-circular exterior of the body portion ofan insert behind the leading insert which is about to be installed,thereby to orientate the engaged insert axially about the mandrel andthereby orientate the leading insert which is about to be installed. 11.An insert installation tool as claimed in claim 10 in which eachsubstantially non-circular shape is substantially square.
 12. A methodof installing an insert in a workpiece, the insert being forinstallation in a relatively thin or weak workpiece to provide an anchorfor a self-tapping screw, which insert is in the form of an integralmetal structure which comprises: a body portion; a head portion locatedat or near one end of the body portion and extending radially therefrom;a receptacle portion located at the other end of the body portion remotefrom the head portion; and a through bore extending throughout the head,body and receptacle portions; the interior bore of the receptacleportion being substantially unthreaded for the reception of aself-tapping screw; the body portion being deformable, when thereceptacle portion is drawn towards the head portion, to secure theinsert in a suitable hole in a workpiece through which the receptacleand body portions have been inserted so that the head portion isadjacent the workpiece and a substantial length of the body portionprotrudes beyond the workpiece; the exterior cross-sectional shape anddimensions of the receptacle portion being smaller than the interiorcross-sectional shape and dimensions of the body portion, such that thereceptacle portion of the insert is a sliding fit within the bodyportion of an identical insert; which method comprises: inserting thereceptacle and body portions of the insert through a suitable hole inthe workpiece so that a substantial length of the body portion protrudesbeyond the workpiece; providing a mandrel having an elongated stem whichextends through the bore of the insert, the mandrel comprising a stemwhich is a sliding fit in the interior of the receptacle portion and aradially enlarged head beyond the receptacle portion; providing atubular interior supporting member circumferentially outside the mandrelstem and inside the body portion of the insert within which is it asliding fit; supporting the head portion of the insert against theworkpiece; withdrawing the mandrel with respect to the head portion ofthe insert so that it engages the receptacle portion of the insert andapplies an axial force to it which causes the body portion to collapseaxially while it is supported internally by the tubular supportingmember.
 13. A method as claimed in claim 12, in which the tubularsupporting member forms part of an installation tool and is re-usable onsuccessively installed inserts.
 14. A method as claimed in claim 12, inwhich the tubular supporting member is provided by the receptacleportion of a further insert through which the mandrel also extends. 15.A method as claimed in claim 12, in which the receptacle portion of theinsert is drawn into the body portion of the insert, the interiorsupporting member being progressively withdrawn as the receptacleportion advances thereby to assist in its deformation and engagementwith the workpiece.
 16. A method as claimed in claim 15, in which themandrel head is drawn completely through the receptacle portion of theinsert, thereby to form the bore through the receptacle portion to apredetermined diameter.
 17. A method of installing an insert as claimedin claim 12 in which both the exterior cross-sectional shape of thereceptacle portion and the interior cross-sectional shape of the bodyportion are substantially square are substantially square.
 18. An insertas claimed in claim 1, in which both the exterior cross-sectional shapeof the receptacle portion and the interior cross-sectional shape of thebody portion are substantially non-circular.
 19. An insert as claimed inclaim 18 in which each substantially non-circular shape is substantiallysquare.